The present invention relates generally to location systems, and in particular, to systems for providing a position of a moving vehicle in an area such as a farm field.
Agricultural practice has typically prescribed that chemicals, fertilizers, seed, and other such crop related substances be applied at one application rate for an entire field. By contrast, modern precision agriculture involves location-specific application of agricultural substances. In precision agriculture, site specific application rates are varied according to locally determined requirements based on the spatial variability of parameters such as soil type, moisture distribution, and fertility. Purely economic pressures and such environmental concerns as reducing pesticide residues on produce and reducing fertilizer leaching in soil are compelling farmers to abandon the one rate per field method of application for the location-specific application method used in precision agriculture.
In precision agriculture, a location system provides information representative of the farm implement position in the field. This position information is typically coupled to a microcomputer which is located on the farm implement. The microcomputer is typically part of a geographic information system (GIS) which stores application rates required for the various locations in the field in a GIS database. An index into the GIS database is formed based on the farm implement position within a specified location. The indexed application rate is retrieved from the GIS database and is used to instruct the implement to apply the prescribed application rate to the field. Such factors as soil acidity, soil type, soil fertility, slope, moisture distribution, elevation, and the like may be considered to determine the prescribed application rates to be stored into the GIS database.
The application rates provided by the GIS may vary in response to information obtained from sensors on the implement itself. These sensors rapidly sense relevant soil and crop parameters and are used to collect data on spacial variability across a field. Data may be collected in real-time to provide real-time control signals for automatic control of farm implement operation. In addition, historical data may be gathered and stored in the GIS database during field operations allowing data from the current and previous seasons to be integrated into the GIS database. In this way, the GIS is utilized to manipulate and overlay data and produce a computerized control map controlling subsequent field operations. The sensors typically sense such parameters as soil moisture content and soil nitrate levels. These types of sensors are used in conjunction with soil maps, manually obtained soil samples for nutrient analysis, and agronomic expertise to develop fertilizer application rate maps which can be stored in the GIS system.
Actuators control the actual application of the agricultural chemicals, fertilizer, and seed based on the rate calculation provided by the GIS system. Both the sensors and the actuators are often controlled with nearby micro-controllers. A controller area network (CAN) standard is being developed to reduce implement wiring requirements and promote inter-operability between agricultural equipment from different manufacturers. In the United States, this CAN standard is an extension of the draft SAE J1939 truck and bus protocol which is described in Society of Automotive Engineers, Recommended Practice for Serial Control and Communication Network (Class C) for Truck and Bus Applications (Draft J1939), Oct. 1, 1992, which is herein incorporated by reference. This protocol is one of three proposals which are currently being suggested to the International Standards Organization which are discussed in Marvin Stone and Mark Zachos, Straight talk: Proposals Identify Common Communication Protocol for Electronics in Agricultural Machines, Agricultural Engineering, Nov. 1992, at 13-16, which is herein incorporated by reference.
Various technological advances have made the variable rate method a more cost-effective alternative to the one rate method of application. Nevertheless, the cost of a location system which provides real-time information representing farm implement position currently makes the variable rate method an expensive alternative for many agricultural applications because of the accuracy required in precision agriculture. Several methods are currently used to obtain real-time farm implement location such as radio beacons and dead reckoning. Dead reckoning is accomplished with such devices as ground speed radar, odometers, speedometers, flux compasses, and the like. Odometer measuring devices, however, do not work well in agricultural applications because of wheel slippage in wet fields.
The global positioning system (GPS) is currently being evaluated as a method to obtain real-time implement location within a field. The GPS system includes a set of satellites put in orbit by the U.S. military which continuously transmit very precise timed radio signals on two frequencies. GPS receivers on mobile vehicles receive these radio signals. A GPS receiver monitors signals from several satellites and calculates the transit time of the signal from each satellite. The distance from each satellite to the receiver is calculated using this transit time information. In this way, the position of the receiver on the mobile vehicle can theoretically be determined by knowing the distance from three satellites. A fourth satellite signal is monitored to determine the altitude of the receiver and to refine the position determination. The receivers' time estimate is adjusted until the calculated distances from the four satellites converge to a single point. If, however, altitude information is known, then only three satellites are needed.
The distance calculation from each satellite to the receiver requires accurate information on the position of a given satellite at a given time. Nevertheless, the satellites are in non-geosynchronous orbits which causes the satellites' position relative to the receiver to change at a rapid rate. Consequently, the radio signals include information that permits the receiver to model each satellite's orbit as a function of time. Ground based radar stations monitor the satellites' orbits and periodically transmit updated orbit information to the satellites. The accuracy needed for the timing information make it cost prohibitive to generate timing information within the receiver so the satellite signals are used to supply the timing information.
Differential GPS utilizes a stationary receiver at a known location to improve the accuracy of the GPS positioning information. The stationary receiver receives signals from satellites and calculates its own position and the distance from each satellite according to the GPS signals. Since the actual position of the base station is known, the errors in the satellite signals are accurately calculated. This error information can be stored for later post processing or transmitted to a mobile receiver over a radio link in real-time. As of Jun. 14, 1993 the GPS system had at least 23 navigational satellites disposed around the earth in such a way that it is possible to take a bearing on and communicate with at least four satellites simultaneously from any position on the earth at any time. By communicating with at least four satellites, a differential GPS system provides a level of accuracy of approximately .+-.10 meters in terms of absolute real-time positional measurement. A more accurate measurement is possible through post-processing the GPS positional data. Post processing, however, is not acceptable during the application of substances onto the field because vehicle position must be known at the time applications are being made.
Because a differential GPS system uses a stationary receiver in conjunction with a mobile receiver. The material cost of the differential GPS system is relatively high as a result of the need for a base station and a communication link to the mobile GPS receiver. Thus, a more cost effective alternative to real time differential GPS is needed for field machines using location-indexed databases for controlling their action.
As mentioned above, the inherit accuracy obtained with a differential GPS system is approximately .+-.10 meters. The military, however, added a selective availability feature to the GPS system to prevent hostile forces from obtaining very accurate position information at a very low cost. When the selected availability is switched on the GPS system accuracy is reduced by the intentional insertion of errors into the satellite orbit information and into the time information in the radio signals from the satellites. When switched on, selective availability degrades the system accuracy to approximately .+-.100 meters. Nevertheless, special military GPS receivers decode information encoded on both the satellite signals to obtain accurate positions even when the selected availability feature is turned on. Presently, selective availability is almost always on, and the military gives no warnings when it does turn off the selected availability feature.